(1) Field of the Invention
The present invention relates to a process and apparatus for learning and controlling the air/fuel ratio in an internal combustion engine. More particularly, the present invention relates to learning the correction control of the air/fuel ratio for each driving region in an electronically controlled fuel supply apparatus having an air/fuel ratio feedback control function.
(2) Description of the Related Art
An air/fuel ratio learning correction control system as disclosed in Japanese Unexamined Patent Publication No. 60-90944 or Japanese Unexamined Patent Publication No. 61-190142 is adopted in certain internal combustion engines provided with an electronically controlled fuel supply apparatus having an air/fuel ratio feedback correction control function.
According to the air/fuel ratio feedback correction control, whether the air/fuel ratio of the practically sucked air/fuel mixture is rich or lean compared to the theoretical air/fuel ratio is indirectly detected based on the oxygen concentration in the exhaust gas detected by an oxygen sensor disposed in the exhaust system of the engine. An air/fuel ratio feedback correction coefficient LMD is increased or decreased and set based on the result of the above detection, and the basic fuel supply quantity is increased or decreased and corrected by this air/fuel ratio feedback correction LMD, whereby the actual air/fuel ratio is feedback-controlled to the theoretical air/fuel ratio.
In this control, the deviation of the air/fuel ratio feedback correction coefficient LMD from the reference value (the value not substantially performing any increase or decrease correction of the quantity of the fuel; for example, 1.0 when the correction coefficient is a multiplier term) is learned for each of a plurality of predetermined driving regions to determine a learning correction coefficient KBLRC, and by correcting the basic fuel injection quantity Tp by the learning correction coefficient KBLRC, the basic air/fuel ratio obtained by the final fuel injection quantity Ti computed without the air/fuel ratio feedback correction coefficient LMD is made substantially equal to the theoretical air/fuel ratio (target air/fuel ratio). Namely, by learning the deviation of the correction coefficient LMD from the reference value, the correction by the correction coefficient LMD is converted to the learning correction coefficient KBLRC, so that the correction coefficient LMD converges on the reference value, and therefore, the target convergent value of the correction coefficient LMD is the reference value.
During the air/fuel ratio feedback control, by performing the correction by the air/fuel ratio feedback correction coefficient LMD, the fuel injection quantity Ti is computed.
By this learning control, correction a meeting the requirements for correction of the air/fuel ratios, differing according to the driving condition, can be performed. Especially, in the case where the required correction value for the air/fuel ratio control is violently changed at the transient driving and there is a response delay in the correction by the air/fuel ratio correction coefficient LMD, the correction corresponding to the driving condition is performed by the learning correction coefficient KBLRC for each driving region and great deviation of the actual air/fuel ratio from the target air/fuel ratio is prevented.
In the low-revolution high-load driving region where hesitation is readily caused, it is more necessary than in other driving regions that hesitation at acceleration should be avoided by controlling the basic air/fuel ratio obtained without correction by the correction coefficient LMD to the rich side. However, in the conventional learning correction control, since such learning that the target air/fuel ratio (theoretical air/fuel ratio) in the air/fuel ratio feedback control can be obtained even without feedback control is not performed through the entire learning driving region, it is difficult to change the learned target air/fuel ratio in a certain driving region, and, therefore, impossible to satisfy the above-mentioned requirement.
More specifically, in the case where it is intended to perform such learning that the target air/fuel ratio is set at a value richer than the target air/fuel ratio (theoretical air/fuel ratio) obtained by the feedback control in a certain driving region, it is necessary to perform learning in this region by practically performing the feedback control to the above-mentioned richer target air/fuel ratio, and during this learning, the target air/fuel ratio by the inherent feedback control cannot be obtained and simultaneously it becomes necessary to detect the air/fuel ratio not only with respect to the target air/fuel ratio by the feedback control but also with respect to the above-mentioned richer learned air/fuel ratio, and therefore, it is impossible to change the target of learning of the air/fuel ratio to a richer or leaner side only in a certain region by simple means.
Because of not only the above-mentioned difference of the required learned target value among the driving regions but also the difference of the properties of the exhaust gas among engines, it is sometimes desired to set the basic air/fuel ratio obtained only by the learning correction without using the feedback correction at a level richer or leaner than the target air/fuel ratio for performing the feedback control, and for the reasons set forth above, this desire cannot be satisfied by simple means.